Handle structs explicitly when adding error type

We want to avoid assuming that all errors reasons are either nil
or structs. This rearranges the clauses and uses pattern matching
to handle the struct case, allowing us to handle anything else
like we handled the nil case before.

lib/spandex_datadog/api_server.ex

@@ -269,8 +269,8 @@ defmodule SpandexDatadog.ApiServer do
   end
 
   @spec add_error_type(map, Exception.t() | nil) :: map
-  defp add_error_type(meta, nil), do: meta
-  defp add_error_type(meta, exception), do: Map.put(meta, "error.type", exception.__struct__)
+  defp add_error_type(meta, %struct{}), do: Map.put(meta, "error.type", struct)
+  defp add_error_type(meta, _), do: meta
 
   @spec add_error_message(map, Exception.t() | nil) :: map
   defp add_error_message(meta, nil), do: meta